Controlled proportional valve

ABSTRACT

A controlled proportional valve is provided, with a main slide valve section (4) containing a main slide valve, which section controls a flow of fluid between a pump connection (P) connected to a pump (2) and a tank connection (T) connected to a tank (6) and two work connections connected to a load (5) and which generates a load-sensing signal (LS INT ) in dependence on the pressures at the work connections (A, B), and with a compensating slide valve section (3) which controls the pressure across the main slide valve section (4) in dependence on the load-sensing signal (LS INT ). It is desirable to achieve a more rapid response time in a proportional valve of that kind, wherein the control means should be capable of being retrofitted to existing proportional valves. To that end, a control arrangement (10) which controls the pressure of the load-sensing signal (LS INT ) to influence the actual volume flow and/or the actual pressure in the work connections (A, B) is provided.

The invention relates to a controlled proportional valve with a mainslide valve section containing a main slide valve, which sectioncontrols a flow of fluid between a pump connection connected to a pumpand a tank connection connected to a tank and two work connectionsconnected to a load and which generates a load-sensing signal independence on the pressures at the work connections, and with acompensating slide valve section which controls the pressure across themain slide valve section in dependence on the load-sensing signal.

EP 0 411 151 A1 describes a proportional valve of that kind in which theload-sensing signal acts on one side of a compensating slide valve. Thepump pressure also acts on that side. The pressure at the output of thecompensating slide valve and an auxiliary pressure changeable betweentwo positions acts on the opposite side of the compensating slide valve.This creates a constant pressure drop across the main slide valvesection. This pressure drop can be changed over between two fixedvalues. In one case, the main slide valve section operates normally. Inthe other case, because of a relatively small pressure drop a moreprecise control is possible, since a more substantial change in theposition of the main slide valve must then be made to achieve the samechange in the volume flow in the work connections. The load-sensingsignal is also additionally fed to a controller, which controls theoutput of the pump.

DE 34 36 246 C2 discloses a control arrangement for a hydraulicallyoperated load, in which the load-sensing signal is no longer solelydependent on the pressure in the work connections, but is formed partlyby the loading pressure and partly by the compensating pressure, thatis, the pressure at the output of the compensating slide valve. In theevent of fluctuations in the loading, the volume flow is then no longerheld constant but drops as the loading increases and increases as theloading decreases. It is intended in this manner to achieve a more rapiddamping of the fluctuations. The pressure of the load-sensing signal isproduced by a pressure divider, the throttle of which is manuallyadjustable, in order to be able to achieve optimal adaptation to eachgiven individual case.

JP 2 262 473 A (Abstract) discloses a hydraulic circuit in which acompensating slide valve is also controlled in dependence on aload-sensing signal. This load-sensing signal is also responsible forregulating the pump output. Part of the load-sensing signal can betapped off and supplied to the other side of the compensating slidevalve.

The present invention is based on the problem of achieving rapidresponse of the proportional valve, wherein it is desirable for thecontrol means used for that purpose to be capable of being retrofittedin existing proportional valves.

This problem is solved in a controlled proportional valve of the kindmentioned in the introduction in that a control arrangement whichcontrols the pressure of the load-sensing signal to influence the actualvolume flow and/or the actual pressure in the work connections isprovided.

By changing the load-sensing signal, the differential pressure acrossthe main slide valve section can be influenced. In this way, the volumeflow through the main slide valve section is influenced, without theposition of the main slide valve having to be changed. Of course, thevolume flow can also still be influenced by a change in the position ofthe main slide valve. Thus, instead of a characteristic that indicatesthe dependency of the volume flow on the position of the main slidevalve, a working range or characteristic range is obtained, since, inaddition to the opening formed by the main slide valve, the pressure canalso be used for control of the volume flow. When the volume flowrequirement is small, the differential pressure across the main slidevalve section can be reduced, which results in a marked reduction inpower loss and thus in an increase in efficiency. When a higher volumeflow is required, it was previously necessary to shift the main slidevalve. But because the main slide valve has a relatively large mass, itsmass inertia prevents a very rapid reaction. This disadvantage can nowbe overcome since the differential pressure across the main slide valvecan be increased very much more quickly so that a rapid change in volumeflow is possible. Because the new control arrangement enables thedifferential pressure across the main slide valve to be increased,existing proportional valves can also be brought up to a substantiallyhigher nominal volume flow. This is especially advantageous when a largevolume flow requirement occurs only briefly.

Advantageously, the control arrangement detects fluctuations in thepressure in the work connections and controls the load-sensing signal incounter-phase to these fluctuations. Such fluctuations are almostinevitable in hydraulic systems since hydraulic systems frequentlyoperate with flexible hoses, which yield slightly under sudden pressurechange and then regain their initial dimension. Such sudden pressurechanges can occur, for example, when loads have to be braked as they arelowered. It was previously not possible to compensate for fluctuationsbecause the inertia of the main slide valve was too great to be able tofollow the rapid fluctuations. The change in the load-sensing signal incounter-phase now enables the pressure across the main slide valvesection to be changed, likewise in counter-phase to the pressurefluctuations in the work connections, which leads to very rapid dampingof these fluctuations.

For that purpose, it is an advantage to provide a pressure-measuringdevice which detects the pressure of the load-sensing signal. Becausethe pressure of the load-sensing signal always detects the pressure inthe work connections, or rather, the higher of the two pressures in thework connections, this feature is sufficient for the pressurefluctuations to be determined effectively.

In an advantageous embodiment, the control arrangement controls also themain slide valve. Changes in the volume flow can then be achieved notonly by changing the pressure across the main slide valve, but also, aspreviously, by changing the position of the main slide valve. This canbe exploited, for example, in that on rapid changes in volume flow thedifferential pressure across the main slide valve is influenced and onslow changes in volume flow the position of the main slide valve isinfluenced. The control arrangement is then able to control the volumeflow in a relatively large region of the characteristic curve.

It is preferable for the control arrangement to control the stationarydifferential pressure across the main slide valve section to achieve thesmallest possible value for the desired or necessary volume flow. Thisleads to a considerable reduction in power loss since the pump then hasto work only at a correspondingly lower pressure. The smallest possiblevalue need not mean the absolute minimum of the pressure difference. Itis quite possible for reserves to be provided so that as a result of arapid pressure change an equally rapid change in the volume flow can beachieved even downwards.

Preferably, on a change in the volume flow the control arrangementchanges firstly the differential pressure across the main slide valvesection in the direction of the volume flow change, and then changes themain slide valve and the differential pressure simultaneously, so that,with the volume flow remaining the same, the smallest possibledifferential pressure across the main slide valve section can be set.This procedure is especially advantageous when a sudden change in volumeflow is followed by a period of uniform volume flow. It is then possibleon the one hand to exploit the advantages of the rapid change, that is,the rapid control of a disturbance, and also on the other hand toexploit the negligible power loss caused by a slight differentialpressure across the main slide valve section.

The control arrangement preferably has a controlled throttling devicewhich connects the load-sensing signal to a pressure source and/or apressure sink. Connection to the pressure source enables the pressure ofthe load-sensing signal to be increased. Connection to the pressure sinkenables the pressure of the load-sensing signal to be reduced. On anincrease in the pressure of the load-sensing signal, simultaneously thepressure difference across the main slide valve section is increased andthe volume flow is enlarged with the position of the main slide valveotherwise unchanged. On a drop in the pressure of the load-sensingsignal, it is the other way round. Because the load-sensing signal canbe changed in both directions, a very wide-ranging control of the volumeflow through the main slide valve section is achieved.

For that purpose, the throttling device preferably has a plus throttlefor increasing the pressure and a minus throttle for reducing thepressure of the load-sensing signal. A controlled increase in thepressure of the load-sensing signal can be effected using the plusthrottle and a controlled reduction of the pressure of the load-sensingsignal can be effected using the minus throttle. The pressure of theload-sensing signal thus be adjusted not only to fixed values, forinstance the pressure of the pressure source or the pressure of thepressure sink, but also to any values between them. The counter-pressurespring of the compensating slide valve can be made smaller or even beomitted. Control of the differential pressure across the main slidevalve section is then effected exclusively under the direction of thecontrol arrangement.

The plus throttle and/or the minus throttle are preferably in the formof pulse width modulated electromagnetic valves. Such valves are veryfast. The pressure of the load-sensing signal is therefore very rapidlyadjusted, which leads to an equally rapid increase in the pressuredifference across the main slide valve. In addition, the technologyknown from controlling the main slide valve can be used to control theload-sensing signal.

The pressure source is advantageously formed by the pump and thepressure sink by the tank. Neither an additional pressure source nor anadditional pressure sink is therefore required. On the contrary,existing arrangements provided in connection with the proportional valvecan be used.

Similarly, to improve the working conditions of the minus throttle, inan advantageous embodiment a pressure regulator that limits thedifferential pressure across the minus throttle to a maximum value canbe provided between the valve arrangement and the pressure sink.

In a preferred arrangement, the differential pressure across the mainvalve can also be controlled either using only the minus throttle and aspring or using only the plus throttle and a spring. When the minusthrottle is used, the spring must be stronger than when the plusthrottle is used. This means that it is possible to omit the respectiveother throttle, which contributes to a simpler construction of theproportional valve.

The invention is described hereinafter with reference to a preferredembodiment and in conjunction with the drawings, in which

FIG. 1 shows a hydraulic system with control of the proportional valve,and

FIG. 2 shows the dependency between the control setting of the mainslide valve and the volume flow.

A hydraulic system 1 is provided, in known manner, with a controllablepump 2 which is connected by way of a compensating slide valve section 3having a compensating slide valve, not illustrated in detail, to a pumpconnection P of a main slide valve section 4 having a main slide valve,also not illustrated in detail. The compensating slide valve and themain slide valve are known per se, see, for example, DE 34 36 246 C2 orEP 0 411 151 A1.

The main slide valve section 4 has two work connections A, B, via whichthe main slide valve section 4 is connected to a diagrammaticallyillustrated load 5, for example a motor. The main slide valve sectionalso has a tank connection T by means of which the hydraulic fluidreturning from the load 5 flows into a tank 6 from which the pump 2 isable to remove the hydraulic fluid again.

At the output of a change-over valve 7, the larger of the two pressuresof the work connections A and B appears on the line 8. This signal isreferred to as a load-sensing signal or load-sensing pressure LS_(INT)and passes to a pressure-measuring device 9 which measures the pressureof the load-sensing signal LS_(INT) and produces from it an electricalsignal which it supplies to a control arrangement 10. Thepressure-measuring device 9 can be, for example, a pressure-to-voltagetransducer. The load-sensing signal LS_(INT) passes by way of a furtherchange-over valve 11, to the other input of which a load-sensing signalLS_(EXT) is fed. At the output of the change-over valve 11, the pressureof the largest of the load-sensing signals is present on the line 12.This signal is referred to as LS_(MAX). The largest of the load-sensingsignals LS_(MAX) is supplied to a pump control device 13 which, by meansof an actuator 14, controls the pump output in dependence on the largestpressure required in the system.

The compensating slide valve section 3 is biased in one direction by aspring 15. The internal load-sensing pressure LS_(INT) present on theline 8 is applied to the same side. On the opposite side, the outputpressure of the compensating slide valve section 3 is fed in, which isat the same time the pressure at the pump connection P of the main slidevalve section 4. Thus, without further measures, a pressure differencewhich is determined by the force of the spring 15 is set across the mainslide valve section 4.

The internal load-sensing pressure LS_(INT) fed to the compensatingslide valve section may, however, be changed by means of a throttledevice which is formed by a plus valve 16 and a minus valve 17. Bothvalves are clocked electromagnetic valves, that is to say, both the plusvalve 16 and the minus valve 17 operate as controllable throttles.

By way of the plus valve 16 the line 8 is connected to the output of thecompensating slide valve section 3. The line 8 can then be connected toa pressure source. As the plus valve 16 opens, an increase in thepressure of the internal load-sensing signal LS_(INT) therefore occurs.Of course, it is also possible in principle to connect the plus valve 16directly to the output of the pump 2. But in that case a relativelylarge pressure difference would be produced by way of the plus valve 16.For clocked electromagnetic valves, as used for the plus valve 16, asmaller pressure difference is, however, better.

The minus valve 17 connects the line 8 by way of a pressure regulator 18to the tank 6. The pressure regulator 18 limits the maximum pressuredifference across the minus valve 17 to a predetermined maximum value.This leads to more favourable working conditions for the minus valve 17.The plus valve 16, the minus valve 17 and the main slide valve section 4are controlled by the control arrangement 10 already mentioned. Thecontrol arrangement 10 may receive an input signal, for example from anoperating device 19, by means of which the volume flow in the load 5 isto be adjusted. It may also receive one or more other external signalswhich can be supplied by way of an input line 20. Finally, as alreadymentioned, it can receive an input signal from the pressure-measuringdevice 9.

The control arrangement 10 detects, for example, fluctuations in thepressure of the internal load-sensing signal LS_(INT). Thesefluctuations are a sign of fluctuations in the work connections A, B,which can arise, for example, when a load has to be suddenly braked asit is being lowered. The control arrangement 10 can now control the plusvalve 16 and the minus valve 17 so that the internal load-sensing signalLS_(INT) fluctuates in counter-phase. This leads to a pressuredifference across the main slide valve section 4 fluctuating incounter-phase, whereby fluctuations in the load 5 are very rapidlyeliminated. It is not necessary to move the main slide valve for thatpurpose. It is sufficient when the pressure difference across the mainslide valve section is varied. But this is easily possible because ofthe rapid reaction times of the plus and minus valves 16, 17 and of thecompensating slide valve section 3.

The control arrangement 10 can also be used to control the volume flowthrough the main slide valve section 4. In order to produce a largevolume flow as rapidly as possible, the plus valve 16 is opened. Thepressure of the internal load-sensing signal LS_(INT) consequentlyincreases. The compensating slide valve of the compensating slide valvesection 3 opens. The pressure difference across the main slide valvesection 4 increases, whereupon a larger volume flow is produced, withoutthe main slide valve having had to move. Conversely, the volume flow canbe reduced just as rapidly by opening the minus valve 17.

This mode of operation is explained with reference to FIG. 2. Here, Qdenotes the volume flow through the main slide valve section 4 and Sdenotes the position of the main slide valve. The curve 21 shows thedependency between the volume flow Q and the positions of the main slidevalve for a conventional proportional valve, that is to say, without thecontrol arrangement 10 and the plus and minus valves 16, 17. In theconventional case, to increase the volume flow from a value Q₁ to avalue Q₂ the position of the main slide valve would have to be movedfrom a position S₁ to a position S₂. In the system illustrated, thepressure is instead increased across the main slide valve section 4, sothat the relationship of the curve 22 is obtained. The position of themain slide valve now needs to be changed only from S₁ to S₃. It isevident that the main slide valve has to cover a substantially shorterdistance. The response time on a increase in volume flow can also bedrastically reduced.

Similarly, to reduce the volume flow from a value Q₂ to a value Q₃, thepressure of the internal load-sensing signal LS_(INT) can be reduced byopening the minus valve 17. The relationship between the position S ofthe main slide valve and the volume flow Q then follows the curve 23.Here too, the main slide valve has to be moved only from position S₃back to position S₁. In the conventional case, it would have to havebeen moved from position S₂ to position S₄.

As readily apparent from the last example, it is also possible to changethe volume flow without moving the main slide valve at all. This ispossible, for example, if it is desired to change the volume flow merelybetween the two values Q₁ and Q₃. For that purpose, it is sufficient forthe pressure of the internal load-sensing signal LS_(INT) to be changedwithout having to move the main slide valve of the main slide valvesection 4. It is therefore possible also to eliminate fluctuations inthe hydraulic system 1, since all that is required is to control thepressure difference across the main slide valve section 4 incounter-phase.

The control arrangement 10 controls not only the plus valve 16 and theminus valve 17, but also the main slide valve section 4. It cantherefore adapt the position of the main slide valve to the pressuredifference across the main slide valve section 4. For example, it canmatch both variables to one another such that for a desired or necessaryvolume flow for the load 5, it is always the smallest pressuredifference across the main slide valve section 4 that is produced. Thisleads to loading on the pump 2 being considerably eased and tonegligible power losses. The smallest pressure difference need not meanthat the absolute minimum is desired. Reserves of control should bepresent so that rapid changes in the volume flow can be effected.

Because the control arrangement 10 controls not only the controlledthrottling device 16, 17 but also the main slide valve section 4, hybridmodes of adjustment can also be implemented. For example, on a change involume flow first of all the pressure across the main slide valvesection can be changed in the direction of the volume flow change. Forexample, the pressure difference across the main slide valve section isincreased when a larger volume flow is required. Once the larger volumeflow has very rapidly been made available, the control arrangement 10 isable to reduce the pressure difference across the main slide valvesection 4 and at the same time change the position of the main slidevalve, the volume flow being unchanged. It is possible to operate with asmall pressure difference across the main slide valve section 4 withouthaving to forgo the advantage of a rapid change in the volume flow.

I claim:
 1. A controlled proportional valve having a main slide valvesection containing a main slide valve, said section having means tocontrol a flow of fluid between a pump connection connected to a pumpand a tank connection connected to a tank and two work connectionsconnected to a load and which generate a load-sensing signal independence on pressures at the work connections, a compensating slidevalve section having means to control pressure across the main slidevalve section in dependence on the load-sensing signal, and including acontrol arrangement having means to control pressure of the load-sensingsignal to influence actual volume flow and/or actual pressure in thework connections.
 2. A proportional valve according to claim 1, in whichthe control arrangement includes means to detect fluctuations in thepressure in the work connections and control the load-sensing signal incounter-phase to these fluctuations.
 3. A proportional valve accordingto claim 2, including a pressure-measuring device having means to detectthe pressure of the load-sensing signal.
 4. A proportional valveaccording to claim 1 in which the control arrangement also includesmeans to control the main slide valve.
 5. A proportional valve accordingto claim 4 in which the control arrangement control means controlsstationary differential pressure across the main slide valve section toachieve the smallest possible value for volume flow.
 6. A proportionalvalve according to claim 5, in which, on a change in the volume flow,the control arrangement control means changes firstly differentialpressure across the main slide valve section in the direction of volumeflow change, and then changes the main slide valve and the differentialpressure simultaneously, and, with the volume flow remaining the same,sets the smallest possible differential pressure across the main slidevalve section.
 7. A proportional valve according to claim 1, in whichthe control arrangement includes a controlled throttling device havingmeans to connect the load-sensing signal to a pressure source and/or apressure sink.
 8. A proportional valve according to claim 7, in whichthe pressure source is formed by the pump and the pressure sink by thetank.
 9. A proportional valve according to claim 7, in which thethrottling device has a plus throttle for increasing the pressure and aminus throttle for reducing the pressure of the load-sensing signal. 10.A proportional valve according to claim 9, in which at least one of theplus throttle and the minus throttle are in the form of a pulse widthmodulated electromagnetic valve.
 11. A proportional valve according toclaim 9, including a pressure regulator having means to limit thepressure drop across the minus throttle to a maximum value, saidpressure regulator being located between the minus throttle and thepressure sink.
 12. A proportional valve according to claim 9, includingmeans to control differential pressure across the slide valve andcomprising the minus throttle and a relatively strong spring.
 13. Aproportional valve according to claim 9, including means to controldifferential pressure across the main valve and comprising the plusthrottle and a relatively weak spring.